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What is the normal water volume per 1 L of blood, and how much deviation from that norm causes dehydration or hyponatremia?

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The human blood contains about 45% of erythrocytes and 54.3% of plasma by volume. The plasma contains about 92% water, while the erythrocytes, about 64% by weight.

The blood is slightly less than 80% water.

Dehydration occurs when water intake is insufficient to replace free water lost due to normal physiologic processes (like sweating, urination etc). Hyponatremia is low sodium concentration in the blood. Usually, the Sodium and water concentrations in blood are regulated independently. Hyponatremia can be caused by intake of to much water, which reduces the sodium concentration.

References: The sodium, potassium, and water contents of red blood cells of healthy human adults by L J Beilin, G J Knight, A D Munro-Faure, and J Anderson, J Clin Invest. 1966 Nov; 45(11): 1817–1825.

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